Many machines, such as engines, motors, compressors and the like, are connected to suitable supports via intermediate mounts. Such mounts are intended to isolate vibrations, but must also be capable of supporting the weight of the machine and damping low-frequency large-amplitude motions of the machine relative to the support (e.g., due to variations in engine speed, load torque reaction, etc.). The design of such mounts is largely dependent upon the nature and types of forces transmitted between the machine and the support. In some applications, such as a gas-powered automobile engine, the mount may simply be an elastomeric block.
In other cases, such as a diesel engine, the mount may take the form of a spring and a damper arranged in parallel with one another to act between the engine and the support. In these latter configurations, the spring is typically made as soft as practicable to support the weight of the engine, but to allow relatively-free vibratory motion of the engine without transmitting large forces through the spring to the supporting structure. The damper is needed to constrain low-frequency large-amplitude transient motions, but inadvertently acts as an unwanted force transmitter at higher frequencies. The reason for this is that the conventional damper typically comprises a piston-and-cylinder arrangement having opposed chambers communicating with one another through a restricted orifice. If there is relative velocity between the engine and support, a pressure differential will be developed across the orifice. This pressure differential acts across the face of the damper piston, and therefore can transmit forces between the engine and the support.
It has been proposed to add "active" elements to such machine mounts. Theoretically, such elements can be selectively controlled so as to effectively cancel the net dynamic force transmitted through the spring and damper due to vibratory motion of the engine. Upon information and belief, it has been heretofore proposed to install an electromagnetic force motor, or "shaker", in parallel with the spring and damper of each mount. An accelerometer mounted on the support in the vicinity of the mount supplies a signal to a controller, which operates the "shaker" to produce an output force waveform on the masses which is of like magnitude but 180.degree. out-of-phase with respect to the sum of the vibrational forces transmitted through the spring and damper, such that the net force transmitted through the suspension is substantially reduced to zero.
The "shaker" must have the capability of overcoming the spring stiffness and damper reaction force at the vibration amplitude determined by the various vibratory forces acting on the engine inertia as a free body in space. The cancelling force which the "shaker" must produce can be reduced by reducing the spring stiffness, but this has the offsetting effect of allowing greater low-frequency spring deflection, for example, during engine torque reaction. The "shaker" motor must therefore be designed to produce the maximum required force anywhere within the spring deflection range. Such "shaker" motors are typically constructed as a conductor (i.e., a so-called "voice coil") moving in a magnetic field. These devices are thus necessarily large, heavy and expensive.
Accordingly, it would be generally desirable to provide an improved machine which avoids the size, weight and cost problems associated with such prior art "shakers", and which is electrically more efficient, while effectively cancelling the transmission of vibrational forces from the machine to the mount.